New Yorkers flocked to a grand piano that someone mysteriously plunked in the East River, staging MTV-style photo shoots and banging on its keys Friday.

“This is a riot. I’m betting an artist put it there, but you kind of wish it just washed up. It’d take a team of people to get it down there, though,” said Muriel Evans, 45, who came to take photos.

The Mason & Hamlin piano — found under the Brooklyn Bridge on the Manhattan side Monday — sits half-submerged at high tide and mostly exposed at low tide.

A passerby stops to take a photo.Photo: William Farrington

It doesn’t work — but that hasn’t stopped musicians from trying.

Amy Kan, 26, of the West Village jumped over a railing to tickle the ivories Friday. Failing to coax out a tune, she said she planned to come back with her cello.

“I stumbled upon it a few days ago and wanted to bring my friend to see it . . . It’s here making people happy. I’m hoping it just showed up,” Kan said.

Others spun whimsical theories about how the 300-odd-pound instrument ended up there.

“Maybe it fell off of a truck from the bridge. Or maybe someone got frustrated learning to play and dumped it in the river, and now it’s here!” said Steve Thompson, 32, of the West Village. “Who knows? It’s fun to think about.”

Some used the piano as a diving board, jumping off it to take photos of themselves mid-leap.

Photo: William FarringtonBrian Smith, 25, of Brooklyn, peeked under the top.

“You think that thing floated from the river? No way! Everything’s still in it. It’s heavy!” he said.

“Someone put it there. And no way did they bring it in from the river. The Coast Guard would have stopped that. Had to be that they came down here and lowered it down.”